Graded Potentials

Graded potentials are changes in membrane potential that are confined to a relatively small region of the plasma membrane and die out within 1 to 2 mm of their site of origin. They are usually produced by some specific change in the cell's environment acting on a specialized region of the membrane, and they are called "graded potentials" simply because the magnitude of the potential change can vary (is graded). We shall encounter a number of graded potentials, which are given various names related to the location of the potential or to the function it performs: receptor potential, synaptic potential, and pacemaker potential (Table 8-3).

FIGURE 8-15

The membrane potential of a cell can be depolarized by using a stimulating current generator, and the potential can be recorded by a pair of electrodes, one inside the cell and the other in the extracellular fluid, as in Figure 8-7.

(a) Membrane potential is closer to the resting potential with increasing distance from the depolarization site.

(b) Local current surrounding the depolarized region produces depolarization of adjacent regions.

Whenever a graded potential occurs, charge flows between the place of origin of the potential and adjacent regions of the plasma membrane, which are still at the resting potential. In Figure 8-15a, a small region of a membrane has been depolarized by a stimulus and therefore has a potential less negative than adjacent areas. Inside the cell (Figure 8-15b), positive charge (positive ions) will flow through the intracellular fluid away from the depolarized region and toward the more negative, resting regions of the membrane. Simultaneously, outside the cell, positive charge will flow from the more positive region of the resting membrane toward the less positive region just created by the depolarization. The greater the potential change, the greater the currents. By convention, the direction in which positive ions move is designated the direction of the current, although negatively charged ions simultaneously move in the opposite direction. In fact, the local current is carried by ions such as K+, Na+, CP, and HCO-T.

Note that this local current moves positive charges toward the depolarization site along the outside of the membrane and away from the depolarization site along the inside of the membrane. Thus it produces a decrease in the amount of charge separation (depolarization) in the membrane sites adjacent to the originally depolarized region.

The voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite the flux due to the concentration gradient affecting that same ion species.

Resting membrane potential = resting potential

The steady transmembrane potential of a cell that is not producing an electric signal.

Graded potential

A potential change of variable amplitude and duration that is conducted decrementally; it has no threshold or refractory period.

Action potential

A brief all-or-none depolarization of the membrane, reversing polarity in neurons; it has a threshold and refractory period and is conducted without decrement.

Synaptic potential

A graded potential change produced in the postsynaptic neuron in response to release of a neurotransmitter by a presynaptic terminal; it may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP).

Receptor potential

A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus.

Depending upon the initiating event, graded potentials can occur in either a depolarizing or hyperpo-larizing direction (Figure 8-16a), and their magnitude is related to the magnitude of the initiating event (Figure 8-16b). Moreover, local current flows much like water flows through a leaky hose. Charge is lost across the membrane because the membrane is permeable to ions, just as water is lost from the leaky hose. The result is that the magnitude of the current decreases with the distance from the initial site of the potential change, just as water flow decreases the farther along the leaky hose you are from the faucet (Figure 8-17). In fact, plasma membranes are so leaky to ions that local currents die out almost completely within a few millimeters of their point of origin. There is another way of saying the same thing: Local current is decremental; that is, its amplitude decreases with increasing distance from the site of origin of the potential. The resulting change in membrane potential from resting level therefore also decreases with the distance from the potential's site of origin (Figures 8-15a and 8-16c).

Because the electric signal decreases with distance, graded potentials (and the local current they generate) can function as signals only over very short distances (a few millimeters). Nevertheless, graded potentials are the only means of communication used by some neurons and, as we shall see, play very important roles in the initiation and integration of the long-distance signals by neurons and some other cells.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.